This application relates to microstructures and nanotechnology.
Microfabrication techniques can be used to fabricate various microstructures on substrates, including micro or nano probes with fine probe tips used in sensing, testing, data memory and other applications. The geometry and dimension of the probe tip, such as the sharpness, size and shape of the probe tip, can affect the performance of a device using such a probe tip, such as the spatial resolution in atomic force microscopy (AFM) imaging and the storage density and operation of a probe-based information storage system. Examples of micro probes and their applications are published in literature, including Rugar et al., Phys. Today 43(10):23-30 (1990); Noy et al., Annu. Rev. Mater. Sci. 27:381-421 (1997); Hansma et al., Annu. Rev. Biophys. Biomol. Struct. 23: 115-139 (1994); Shao et al., Quart. Rev. Biophys. 28:195-251 (1995); Binnig et al., U.S. Pat. No. 5,835,477; and Vettiger et al., J. Microelectron. Eng. 46:11-17 (1999).
Some commercially available AFM probe tips are made of silicon or silicon nitride (Si3N4) which is microfabricated into a pyramid configuration. Such probes can be made to have a tip radius of curvature in the regime of about 50 nm regime and exhibit a limited lateral resolution. The rigid pyramid shape can be difficult to access to narrow or deep structural features.
Advances in carbon nanotube (CNT) science and technology now allow carbon nanotubes to be used in micro probe tips. One example of a carbon nanotube probe uses a “thin-probe-on-pyramid” configuration where a carbon nanotube is formed on a pyramid base as the probe. See, for example, U.S. Pat. Nos. 6,716,409 and 6,401,526; Dai et al., Nature 384:147-150 (1996); Colbert et al., Science 266: 1218-1222 (1994); Wong et al., J. Am. Chem. Soc. 120:603-604 (1998); Nishijima et al., Appl. Phys. Lett. 74:4061-4063 (1999); Stevens et al., Nanotechnology 11:1-5 (2000); Yenilmez et al., Appl. Phys. Lett. 80:2225-2227 (2002); and Minh et al., J. Vac. Sci. Technol. B21(4):1705-1709 (2003)). Carbon nanotubes can be grown by using the chemical vapor deposition (CVD) technique in which hydrocarbon gas is decomposed at a high temperature often assisted by DC or RF plasma.
The long and slender geometry of carbon nanotubes provides a high aspect ratio and can be advantageously used for probing narrow and deep features. The elastically compliant behavior of high aspect ratio nanotubes may be beneficial in certain applications where the probe tip can directly touch a surface and bend without being damaged or damaging the surface in contact when the contact force is within a certain limit. Even when the stress encountered by the nanotube probe reaches beyond the limit or a critical force, the nanotube can elastically buckle and recover to accommodate the strain, thus limiting the maximum force exerted onto a sample being imaged by the AFM probe. This feature of the CNT probe tip can be useful when the samples being examined are mechanically soft or fragile such as some biological surfaces. The CNT can be attached to an AFM probe tip by several different means, for example, using acrylic adhesives under optical microscope, carbon deposition in a scanning electron microscope (SEM), or electric arc discharge technique. In situ growth of carbon nanotubes directly on AFM tips were also reported in U.S. Pat. Nos. 6,716,409 and 6,401,526, Yenilmez et al. and Minh et al.